1. Field of the Invention
The present invention relates generally to a device to linearly illuminate an object. Furthermore, the invention concerns a method to linearly illuminate an object and a scanning system.
2. Description of the Related Art
A typical use for a device to linearly illuminate an object is an image acquisition system, in which a sensor (for example a camera) detects graphical elements of a master, digitalizes these graphical elements, and processes the image data. The quality of an image acquisition system is directly dependent on the amount of light that is available to the sensor during the scanning to generate the corresponding signals. It is therefore desirable (in particular in the field of rapid line-scanning systems) to provide a large amount of light, since a higher signal-to-noise ratio is thereby supplied for the subsequent signal processing. Furthermore, it is necessary to employ a light source with as little power dissipation as possible that exhibits a high time stability and temperature stability.
Light-emitting diodes, what are known as LEDs, have very good characteristics for use in image acquisition system with regards to lifespan, time stability, temperature stability, and efficiency of the light yield. Known scanning systems that use LED lines focus the radiation with the aid of bar lenses or Fresnel lenses. Such systems are used for monochromatic light.
Due to newer developments, polychromatic LEDs are currently available that also emit white light. However, such LEDs have a smaller light-power density, such that they can only be employed in a very restricted manner in image acquisition systems. The use of lens components, for example of bar lenses or Fresnel lenses, in polychromatic light leads to significant chromatic image errors. These chromatic image errors manifest themselves in color errors on the focus edges. However, in the scanning of color graphic image elements it is necessary to have a homogenous color temperature for the entire field of illumination in order to prevent an incorrect scan. Therefore, it is only possible with great effort to focus light radiation of polychromatic LEDs with the aid of lens systems.
An optical scanning system is known from U.S. Pat. No. 5,828,050 that operates with a curved pivotable mirror. The radiation of a monochromatic LED is directed by the mirror as a light spot on to an object to be scanned. The received beam path of a light-receiving diode is directed via the same mirror, such that the graphical elements illuminated in the light spot are scanned in the pivoting movement of the mirror.
A scanning system with a mirror arranged in stationary fashion is specified in U.S. Pat. No. 5,412,205 that is elliptical in cross-section. An elongated fluorescent tube is arranged in the first focal line of the mirror, the document to be scanned in the second focal line. The radiation of the fluorescent tube is focused by the mirror linearly onto the document.
An image reading device is known from Japanese Patent Document JP 08-307610A that uses an LED as a light source. This LED has an elliptical mirror, whereby the LED is arranged in a focal point of the mirror and the other focal point of the mirror is arranged on the image to be read.
Furthermore, reference is made to the following prior art: Japanese Patent Document JP 2000-312 305 A, French Patent Document FR 1 237 467, German Patent Document DE-OA 2 217 421, U.S. Pat. No. 6,133,565, Japanese Patent Document JP 56-106 257 A with abstract, German Patent Document DE 195 32 877 A1, and German Patent Document DE 42 23 916 C2. The use of elliptical mirrors in connection with digital cameras and scanning devices is specified in these documents.